Abstract

The Ebro river basin, in the northeastern part of the Iberian Peninsula in Europe, very often experiences radiation fog episodes in winter that can last for several days. The impact on human activities is high, especially on road and air transportation. The installation in July 2009 of a WindRASS in the area, which is able to work in the presence of fog, now allows inspecting the vertical structure of the temperature and wind profiles across the roughly 300-m-thick fog layer. We present a case study of a long-lasting (60 h) deep radiation fog that took place in December 2009 to obtain a deeper understanding of the dynamic processes governing such persistent fog. Field observations of vertical profiles of temperature, wind and turbulent kinetic energy are compared with a high-resolution mesoscale simulation, satellite imagery of fog distribution and observations taken in the area to understand why the fog is so persistent and how it dissipates only for a short period in the afternoon despite intermittent turbulence within the fog deck. The confinement of the fog inside a practically closed basin allows us to study the relevant physical processes in the establishment and subsequent evolution of the fog episode using a limited-area mesoscale model. The contribution of the WindRASS measurements allowed us to validate the numerical simulations, particularly inspecting the role of turbulence that can link the bottom and top of the fog through moderate episodic mixing. The fog layer has very weak winds inside, but is well mixed and experiences intermittent top-bottom turbulence generated in its upper part by convection due to radiative cooling and by wind shear due to the topographically generated flows that blow just above the top of the fog.

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Acknowledgements

ECMWF and AEMET are thanked for the access to computing time. We also thank Jordi Cunillera and Antonio Gázquez (SMC) for access to data from the SMC Network and from the special devices installed in Raimat, Armand Alvarez (AEMET) for his support and knowledge of the local meteorology, Felipe Molinos for the initial treatment of the WindRASS and fog data, Daniel Martinez for insightful comments on the manuscript, and the Meso-NH team in Meteo France and Laboratoire d’Aérologie. This work was partially funded by SMC and through project CGL2009-12797-C03-01 of the Spanish Government also supplied with European FEDER funds. Data from MODIS are distributed by the Land Processes Distributed Active Archive Center (LPDAAC) located at the US Geological Survey (USGS) Earth Resources Observation and Science (EROS) Center (http://www.lpdaac.usgs.gov).